fix kokoro path (#67)

Taking a stab in the dark as to where the repository is located, based on
https://github.com/googleapis/google-cloud-java/blob/940999d7a50b1d006b5869c1bcd1eea7aaec4c71/.kokoro/common.cfg#L12
diff --git a/internal/kokoro/test.sh b/internal/kokoro/test.sh
index a116c4c..6446d94 100755
--- a/internal/kokoro/test.sh
+++ b/internal/kokoro/test.sh
@@ -9,7 +9,7 @@
 set -x
 
 # cd to project dir on Kokoro instance
-cd git/gax-go
+cd github/gax-go
 
 go version